* Air Pollution, Climate Change and Ozone Depletion  Notes

Box 2 - Acid Rain

Sulphur (SO2) and nitrogen (NO and NO2) oxides are emitted from power stations and factories. Once in the atmosphere gas-phase and aqueous-phase oxidation of these oxides can occur producing acids. For example, in the gas-phase SO2 can be oxidised by the OH free radical to form HSO3, which is rapidly converted to SO3 via reaction (2) and subsequently taken up by water in the atmosphere to form sulphuric acid.

SO2 (g) •OH (g) → •HSO3 (g) 1
•HSO3 (g) + O2 (g) → •HO2 (g) + SO3 (g) 2
SO3 (g) + H2O (l) ® H2SO4 (aq) 3

Aqueous oxidation can occur because SO2 can dissolve in cloud droplets directly and can then be oxidised in solution to sulphuric acid

uptake in cloud droplets
SO2 (g) → SO2 (aq)

aqueous oxidation by hydrogen peroxide dissolved in cloud droplets
SO2 (aq) + H2O2 (aq) ® H2SO4 (aq)

aqueous oxidation by ozone dissolved in cloud droplets
SO2 (aq) + O3 (aq) + H2O (l) ® H2SO4 (aq) + O2 (g)

It is estimated that between 48 and 84 % of SO2 oxidation occurs via oxidation in the aqueous phase.

See http://schools.ceh.ac.uk/basic/acidrain/acidrain1.htm for more information.